1. Field of the Invention
The present invention relates to an image processing method and apparatus and to an image forming method and apparatus, and more particularly, to half-toning technology used in an image forming apparatus of a dot recording system, which is typified by an inkjet printer.
2. Description of the Related Art
An image forming apparatus has been known which forms an image on a recording medium by means of dots, such as an inkjet printer, a thermal printer, and an LED printer, and the like. In an image forming apparatus of this kind, an image is formed by means of a plurality of dots formed by liquid ink droplets, toner particles, or the like, and therefore, essentially, an image is reproduced by depositing or not depositing dots at locations on a recording medium, such as a white paper. Since the number of inks available is limited, a half-toning method has been known which is used to represent continuous tonal graduation by means of a limited number of inks, or the like.
As one method for converting multiple-value image data which has a multiple-value graduated tone value for each pixel, into binary image data, Japanese Patent Application Publication No. 2004-142196 discloses a method which combines a density pattern method with an error diffusion method. In order to prevent decline in density due to defective nozzles, Japanese Patent Application Publication No. 2004-142196 proposes technology which restricts the quantization number used in the error diffusion method in order to avoid the use of nozzles suffering ejection failure.
Japanese Patent Application Publication No. 2001-054956 discloses a method in which, in order to prevent cyclical (spatially-periodic) non-uniformities which are a distinctive feature of a density pattern method, a dot pattern table which is expanded two-dimensionally is used to select the dot pattern to be adopted for each graduated tone value, on the basis of the positional information relating to the pixel.
Japanese Patent Application Publication No. 2004-058282 discloses technology whereby, in order to correct image defects, such as banding caused by variation in the volume or the ejection direction of a ink droplet ejected from one of the respective nozzles, a shading correction is applied by multiplying a correctional coefficient with respect to the processed nozzle and the surrounding nozzles thereof.
However, in the method disclosed in Japanese Patent Application Publication No. 2004-142196, it is difficult to correct for the banding caused by recording errors other than ejection failure, in other words, it is difficult to correct for the banding caused by variation in the ejected volume of the ink droplet or the ejection direction. Furthermore, the technology described in Japanese Patent Application Publication No. 2004-142196 does not include the concept of using the nozzles surrounding a nozzle suffering ejection failure in order to perform correction, and the corrective performance of this technology is low.
Although the technologies described in Japanese Patent Application Publication No. 2001-054956 and Japanese Patent Application Publication No. 2004-058282 remedies the drawbacks of Japanese Patent Application Publication No. 2004-142196, they involve problems of the following kinds, when considering their application to a combination of an error diffusion method and a density pattern method. Here, with regard to the terms used in the following description, when recording is carried out by moving the recording head and the recording paper (recording medium) relative to each other, the direction of this relative movement is called the “sub-scanning direction” and the direction perpendicular to this sub-scanning direction is called the “main scanning direction”.
The problems that arise when carrying out the technologies disclosed in Japanese Patent Application Publication No. 2001-054956 and Japanese Patent Application Publication No. 2004-058282 are as follows. In these technologies, a plurality of dot pattern tables used to correct the nozzle characteristics are prepared respectively for the graduated tone values, and each of the dot pattern tables has a size in the main direction of a number of dot pixels corresponding to the number of recording elements and a size in the sub-scanning direction of a prescribed number of dots (a sufficiently large number of dots). A dot pattern is then selected from the dot pattern tables on the basis of the graduated tone value of the corresponding pixel and the positional relationship of the pixel (i.e., the position in the main scanning direction (the position of the nozzle used) and the position in the sub-scanning direction), and an image is formed in accordance with the selected dot pattern. In the image processing, the dots in the end portions of the dot pattern are determined by incorporating the effects of dots belonging to neighboring pixel blocks (dot patterns). Therefore, although there is no problem if the neighboring block and the block under processing have the same graduated tone value, the corrective performance will decline in the dot pixels belonging to the boundaries of a block, if the neighboring blocks have mutually different graduated tone values. Since dot pixels suffering this decline in corrective performance (namely, pixels in the boundary sections of blocks) are arranged consecutively in the sub-scanning direction and are also aligned in a regular fashion in the main scanning direction, then there is a problem in that cyclical non-uniformities caused by these dot pixels suffering the decline in corrective performance are liable to be visible.